The arch of the foot is important in performing activities to provide a stable base for weight bearing, flexibility to accommodate different ground conditions, and also to dissipate loads from weight bearing. The arch is constrained by bony and articular geometry, ligaments, joint capsules, and fascia. Posterior tibial tendon dysfunction (PTTD) is associated with loss of these constraints, leading to pain, impaired function, and flatfoot deformity. This study will focus upon four specific aims related to function of the arch and to mechanical behavior of the foot with orthoses or reconstruction procedures for PTTD and flatfoot. Hypothesis 1: Normal gait may be observed in vitro with a dynamic ankle simulator. Specific Aim 1: To determine foot and ankle kinematics, ground reaction force, and center of pressure advancement in level walking in normals. Hypothesis 2: Posterior tibial tendon dysfunction and flatfoot are associated with markedly abnormal foot and ankle kinematics, ground reaction force, and center of pressure advancement in level walking. Specific Aim 2: To determine foot and ankle kinematics in cadaveric feet with simulated posterior tibial tendon dysfunction and flatfoot and compare results with normals. Hypothesis 3: Hindfoot orthoses (HFO) improve foot and ankle function in feet with posterior tibial tendon dysfunction and flatfoot on level walking. Specific Aim 3: To evaluate mechanical behavior of the foot and ankle with posterior tibial tendon dysfunction and flatfoot treated with articulated and nonarticulated HFO on level walking. Hypothesis 4: Reconstruction operations for posterior tibial tendon dysfunction and flatfoot which do not involve extensive arthrodesis improve foot and ankle function in level walking. Specific Aim 4: To evaluate mechanical behavior of the foot and ankle with three recently developed and yet to be proven operations for posterior tibial tendon dysfunction and flatfoot: lateral column lengthening (LCL), medial displacement calcaneal osteotomy (MDO), and Giannini subtalar implant.